The aim of this work is to continue the study of the mechanisms of urea formation in a variety of human tissues obtained from normal controls and from patients with disorders of the urea cycle. The objectives are to search for isozymes of argininosuccinate synthetase in control human liver and kidney by disc gel electrophoresis and isoelectric focusing; to purify and characterize argininosuccinate synthetase from control human liver and kidney and to characterize the enzymes in terms of kinetic parameters, heat stability, molecular weight, sub-unit structure, isoelectric point, amino acid composition and immunological specificity. The specific antisera will be used to identify total of partial cross-reactivity of the two enzyme preparations, and to study cross-reactivity with enzyme from a variety of control human tissues as well as from fibroblast extracts derived from patients with citrullinemia. A second objective is to search for complementation in heterokaryons derived from all possible paired combinations of fibroblasts from eight patients with citrullinemia. Complementation will be assessed by the ability of heterokaryons to incorporate 14C-citrulline into trichloroacetic acid-precipitable material. Such studies will provide information on the nature of the genetic lesion in these patients.